The invigoration and relief found after an enema can be caused by "stimulation" rather than anything else, and as with any "forcing measures". Stimulation wastes the life-force, which is the energy we are trying to recuperate on a fast, rather than dissipate.
It is known that any impacted fecal matter within the bowels/colon is Autolyzed, or used as fuel within the first 10 days of a water-only-fast.